Sylvester Stallone Says Conor McGregor Would Be "Foolish" To Turn Down Jake Paul Fight

Sylvester Stallone says Conor McGregor would be FOOLISH to say no to a Jake Paul fight ... telling TMZ Sports the money would be INSANE!!

We talked to Stallone -- a HUGE boxing fan -- outside Cafe Roma in Beverly Hills ... and asked the 'Rocky' star whether McGregor should take the bait and fight the YouTuber turned boxing star.

"Are you kidding. With the kind of money that's going around, he'd be foolish not to. Think about it."

Of course, Conor and Jake have been verbally sparring on social media and in the media ... but when McGregor was recently asked about fighting JP, he didn't exactly say no.

"I don’t see so, but never say never," Conor said. "If they’re gonna keep competing and what not, who f***in' knows? But dingbats, the 2 of 'em."

For his part, Jake offered CM $50 million for a fight earlier this year.

But, assuming they did fight ... Conor is a former 2-division UFC champion, and Jake is relatively new to the sport (3-0) -- so, does Paul have a real shot at beating The Notorious??

"Oh yeah," Stallone says ... "[Jake's] got range, and he's got youth. And, he's not easy to knockout."